Replace os.path with pathlib

[`pathlib`](https://docs.python.org/3.8/library/pathlib.html) is the standard and preferred way to manipulate paths. We should migrate `os.path` calls to `pathlib`.

References:

- https://docs.python.org/3.8/library/pathlib.html
- https://treyhunner.com/2018/12/why-you-should-be-using-pathlib/